A hybrid vehicle powertrain includes an engine and an electric motor, wherein torque (or power) produced by the engine and/or by the motor can be transferred to the vehicle drive wheels through a transmission. A traction battery or other electrical power source connected to the motor supplies energy to the motor for the motor to produce the motor torque. The motor may provide a negative motor torque to the transmission (for example, during regenerative braking) and thereby act as a generator to store electrical energy in the battery.
In a configuration sometimes referred to as a modular hybrid transmission (“MHT”) configuration, the engine is connectable to the motor by a disconnect clutch and the motor is connected to the transmission. The motor may be connected to the transmission via a torque converter having a torque converter clutch. The engine, the disconnect clutch, the motor, the torque converter, and the transmission are connected sequentially in series.
Operation with the torque converter bypass clutch disengaged provides a hydrodynamic coupling that provides torque multiplication and damping of driveline disturbances, but also reduces energy efficiency of the vehicle. Engagement or locking of the torque converter bypass clutch eliminates the hydrodynamic coupling and torque multiplication of the torque converter to reduce losses and improve energy efficiency, but may transmit torque disturbances from the engine or the motor through the driveline. As such, it is desirable to control the torque converter bypass clutch position to operate in a reduced slip, disengaged position. Modulation of the torque converter bypass clutch to operate in a reduced slip position near engagement reduces losses and contributes to improved energy efficiency while also providing some damping of driveline disturbances. However, various vehicle operating conditions may result in the torque converter slip approaching zero and the torque converter transmitting driveline torque disturbances, such as torque reversals associated with accelerator pedal tip-out and vehicle coasting or braking, for example.